CVE-1999-0230

Step 1: Target Identification: The attacker identifies a Cisco 7xx router accessible via Telnet (port 23).

Step 2: Payload Crafting: The attacker creates a malicious payload, including a long string designed to overflow the buffer and overwrite the return address on the stack. This payload may also include shellcode to execute arbitrary commands.

Step 3: Payload Delivery: The attacker connects to the router's Telnet service and sends the crafted payload.

Step 4: Buffer Overflow Trigger: The Telnet service receives the oversized input, and the buffer overflow occurs within the vulnerable function.

Step 5: Code Execution: The overflow overwrites the return address, causing the program to jump to the attacker-controlled memory location, executing the shellcode (or other malicious code).

Step 6: System Compromise: The attacker gains control of the router, potentially gaining access to the network, modifying configurations, or intercepting traffic.

The vulnerability lies within the Telnet service's handling of user input. Specifically, the service fails to properly validate the size of data received through the Telnet protocol. This allows an attacker to send an overly long string, exceeding the allocated buffer size within the Telnet service's processing logic. This buffer overflow overwrites adjacent memory locations, potentially overwriting critical program data or control flow elements. By carefully crafting the malicious input, an attacker can overwrite the return address on the stack, redirecting program execution to arbitrary code, such as a shellcode payload. The root cause is a lack of bounds checking on the input received via Telnet, leading to a classic stack-based buffer overflow.